1. Field of the Invention
The present invention relates to alignment of substrates for bonding.
2. Description of Related Art
Substrate bonding is a technology used in micro-electronics fabrication, in which a first substrate carrying first devices on its surface is brought into contact with second devices on a surface of a second substrate for fabricating an electronic circuit. Typically, the contact is arranged in such a way that signals can be transported from at least one first device on the first substrate to at least one second device on the second substrate or vice versa. The signals to be transported between the first device and the second device may be either electronic, photonic, fluidic, gaseous, magnetic, or sonic, or may relate to any other conceivable type of signal. Substrate bonding is used in instances in which manufacture of a device as a single entity is too expensive, impractical, or is not possible.
An example where substrate bonding is useful in reducing costs is when Si-based first devices need to be combined with second devices based on another substrate material such as a III-V or II-VI semiconductor substrate. Manufacturing of both types of devices on a single substrate may be virtually impossible due to the incompatibility of the constituent materials.
Another example is an electronic circuit made out of a first and second substrate where, for reason of complexity, devices need to be fabricated on separate substrates (even though both substrates may be based on a similar substrate type).
In cases where an electronic circuit or “chip” needs to combine two or more devices not manufactured on a single substrate, substrate bonding can provide a way to combine those devices into a single package with a desired (electronic) function. On each device, areas are reserved for providing one or more contact pads. A contact pad is intended to provide a connection between one of the devices and a similar contact pad on the other of the devices. Typically, the location of each contact pad within the device area is defined during the design of the device or the electronic circuit of which the device is a part. The contact pads on devices to be bonded must have a sufficient alignment and overlap (i.e., a coincidence in their respective lateral positions within the device area) that a functional contact between the first and second devices is achieved; i.e., the electronic circuit formed from the devices is actually functional.
For circuits where signals other than electric signals need to be exchanged between the first and second devices, contact openings (for channels guiding the signals) may be provided instead of contact pads. In that case, the bonding operation is performed so as to align the contact openings on first and second devices with respect to each other, and thereby achieve functional contact between the first and second devices.
Conventional substrate bonding machines may provide a substrate bonding capability with a limited accuracy, typically about 100 micrometers. In order to compensate for this limited accuracy, large contact pads are provided on the first and second devices (to ensure that the contacts connect with each other when the devices are bonded together). This can be disadvantageous because the contacts take up a large amount of the available area on the substrate.
Moreover, conventional substrate bonding machines provide alignment of first and second substrates in a mechanical manner, without taking into account the locations of devices on the substrates. The mechanical alignment between substrates is assumed to coincide with the alignment of devices on the substrates. If smaller contacts were to be used, then this approach may become unacceptably inaccurate.
An alternative, currently used approach to achieve alignment is to direct infrared light through substrates, to allow inwardly facing alignment marks to be viewed. A disadvantage of this approach is that the substrate must be transparent to infrared light.
A further alternative, currently used approach to alignment is to align a first substrate using a first detector, move that substrate out of the way whilst monitoring its location, then align a second substrate to the previously measured position of the first substrate. The first substrate is then returned to its original aligned position, so that the first and second substrates are aligned relative to one another. This approach is slow and requires expensive location monitoring equipment.